This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to performance characterization of MRI systems including measuring the spatial distribution of the system output and determining the actual system response following the application of a magnetic field gradient waveform.
The ability to measure gradient fidelity can be important in several applications. Image acquisition and RF excitation techniques generally assume that the desired time-varying gradient fields and resulting k-space trajectories can be faithfully created on the magnetic resonance system. Problems can arise from a variety of effects, such as amplitude tracking errors, temperature-related drifts, and system instabilities. As a result, reconstructed images and excitation profiles can suffer from degradations in quality A. Takahashi, T. Peters, "Compensation of multidimensional selective excitation pulses using measured k-space trajectories", Magn. Reson. Med. 34:446-456 (1995); D. M. Spielman, J. M. Pauly, "Spiral imaging on a smallbore system at 4.7 T, Magn. Reson. Med., 34:580-585 (1995); A. B. Kerr, J. M. Pauly, C. H. Meyer, D. G. Nishimura, "Image quality for spiral-based sequences", Proc., SMR, 3rd Annual Meeting, Nice, p. 622 (1995); and W. Block, J. Pauly, A. Kerr, D. Nishimura, "Improved spectral-spatial pulses through B.sub.o eddy current compensation", Proc., ISMRM, 4th Annual Meeting, New York, p. 196 (1996). Further errors can occur if any residual gradients are present after the desired waveforms have terminated. For example, the strong gradients used in diffusion-weighted imaging can produce eddy currents which can cause undesired phase shifts and even distortions and aspect ratio changes in the final images K. Butts, A. de Crespigny, J. M. Pauly, M. Moseley, "Diffusion-weighted interleaved echo-planar imaging with a pair of orthogonal navigator echoes", Magn. Reson. Med. 35:763-770 (1996); and J. C. Haselgrove, J. R. Moore, "Correction for distortion of echo-planar images used to calculate the apparent diffusion coefficient, Magn. Reson. Med. 36:960-964 (1996). Finally, unwanted phase effects can occur even under ideal conditions. Any gradient will produce concomitant Maxwell gradients that, if strong enough, can cause distortions in off-isocenter images D. G. Norris, J. M. S. Hutchison, "Concomitant magnetic field gradients and their effects on imaging at low magnetic field strengths", Magn. Reson. Imaging, 8:33-37 (1990); R. M Weisskoff, M. S. Cohen, R. R. Rzedzian, "Nonaxial whole-body instant imaging", Magn. Reson. Med. 29:796-803 (1993); and M. A. Bernstein, X. Zhou, K. F. King, A. Ganin, N. J. Pelc, G. H. Glover, "Shading artifacts in phase contrast angiography induced by Maxwell terms; Analysis and Correction", Proc., ISMRM, 5th Annual Meeting, Vancouver, p. 110 (1997).
Several procedures have been used to measure gradient behavior on a Magnetic Resonance (MR) system. Eddy current characterization can be done with the "grafidy" technique, which uses small test samples and a strong test gradient followed at various delays by an RF pulse G. H. Glover, N. J., Pelc, "Method for magnetic field gradient eddy current compensation", U.S. Pat. No. 4,698,591 (1987) and G. H. Glover, N. J. Pelc, K. M. Bradshaw, "Gradient and polarizing field compensation", U.S. Pat. No. 4,950,994 (1990). The resulting FID is then analyzed to measure the residual fields. The "peak-fitting" technique for measuring k-space trajectories, a method which is related to the present invention, analyzes the echo peaks produced by the combination of so-called self-encoding gradients and a test waveform A. Takahashi, T. Peters, "Compensation of multidimensional selective excitation pulses using measured k-space trajectories", Magn. Reson. Med. 34:446-456 (1995); T. Onodera, S. Matsui, K. Sekihara, H. Kohno, "A method of measuring field-gradient modulation shapes. Application to high speed NMR spectroscopic imaging", J. Phys. E. Sci. Instrum., 20:416-419 (1987); and A. Takahashi, T. M. Peters, "Measurement of k-space trajectories for RF pulse design", Proc., SMRM, 12th Annual Meeting, New York, p. 424 (1993).
The present invention is directed to an alternative procedure for measuring the magnetic field gradient produced by an arbitrary gradient waveform.